Novolacs, the acid-catalyzed condensation products of phenols with less than 1 mole of formaldehyde per mole of phenol, constitute a well-known class of phenolic resins and have found wide use. The novolacs in commercial use are derived preponderantly from mononuclear, monohydric phenols, such as phenol itself, cresol, p-t-butylphenol, octyphenol and xylenol. Large amounts of resorcinol, a mononuclear, dihydric phenol, are also used, however, in novolac manufacture.
A relatively recently developed novolac is derived from bisphenol A and contains eight aromatic nuclei (benzene rings), each having a phenolic hydroxyl attached to it. This novolac is marketed by Celanese Corporation in the form of its octaglycidyl ether, a high molecular weight epoxy resin known by the tradename "SU-8". The (unadvanced) novolac may be represented by the following ideal structure: ##STR1## It will be seen that the novolac of formula (1) is actually linear in structure, i.e., is not branched.
SU-8 is a hard, high melting epoxy resin which reacts rapidly, has good heat resistance and cold-flows to a lesser extent than more conventional high molecular weight epoxy novolacs. However, it not only is brittle but also contains low molecular weight epoxide species derived from the free bisphenol inevitably present in novolacs prepared by the reaction of formaldehyde with bisphenols. The latter species are present in sufficient amount so that the epoxy will still cold flow, albeit to a limited extent. Also inevitably present in conventional bisphenol novolacs are species of sufficiently high molecular weight to undesirably increase the gelling speed of the "epoxidized" novolac.
Thus, it is clear that a method of making bisphenol-derived novolacs which largely retain the advantages but not the disadvantages of the novolac precursor to SU-8 would be highly desirable.